1. Field of the Invention
The present invention relates to a medical display and a medical display system using the display. In particular, the present invention relates to a medical display provided with an anti-reflection film which is flat or has a given degree of flatness, in other words, which does not have an anti-glare property, and a medical display system for displaying an image on the medical display.
2. Description of the Related Art
A diagnostic image taken and processed by a medical measurement (image pickup) apparatus such as an MRI, a CT scanner, a DSA apparatus, an FCR (Fuji computed radiography) or other CR apparatus, a mammography apparatus, or a digital X-ray radiography (DR) apparatus, is usually recorded on a light-transmissive image recording film such as an X-ray film or a photosensitive film, and reproduced as a light-transmissive image. The film on which the diagnostic image is reproduced is set in a light source apparatus called a film viewer and is illuminated from the back. The backlit diagnostic image on the film is observed as a transmission image for diagnosis.
An alternative has become available in recent years and a diagnostic image taken by a medical measurement apparatus can now be displayed for diagnosis on various image display devices (electronic film viewer) such as a liquid crystal display device (LCD), a plasma display panel (PDP), an electroluminescence display (ELD, in particular, organic ELD), or a CRT (cathode ray tube) display device.
In an image display device (display) as given above, its display screen has an anti-glare (AG) property in order to avoid surface reflection on the display screen and to prevent an image displayed on the screen from glaring for improved visibility of the display screen. The AG property is obtained by making the front or rear surface of the material of the display screen, for example, a glass substrate or a polarization plate, irregular and then matting the front and rear surfaces of the substrate or the polarization plate through application of an application solution that contains matte particles. Alternatively, the AG property is acquired by forming transparent films on the front and rear surfaces of a substrate or a polarization plate and making the surfaces of the transparent films irregular through embossing or other method (See JP 2000-275404 A).
Giving the AG property to a display screen (polarization plate) of the display is effective against reflection light of an observer or an external object on the display screen.
On the other hand, the vividness and sharpness of an image displayed are degraded because of the AG property, resulting in a blurred image. Also, glaring becomes conspicuous in medical displays of 100 to 300 ppi (pixel per inch), in particular, LCDs having a 200 ppi or higher resolution matrix structure for interpretation of a mammographic image in which minute calcification or the like has to be interpreted. If the medical display in question is a monochrome display, image disturbances are particularly noticeable, and an image is shifted in the front-to-back direction due to the AG property, thus giving a ‘double vision’ look to the image. The image disturbances cause an eye strain and present difficulties in interpreting the image.
In addition, the AG property makes the display screen reflect light from the surroundings at an increased diffuse reflectance. The screen therefore assumes a whitish appearance, causing dark areas of a displayed image to look protruded and lowering contrast. A medical image generally has a wide dynamic range of shades, so that lowering of contrast presents a great deterrent in making a diagnosis.
A transmission image on a film to be interpreted for diagnosis by a conventional film viewer shows a slight difference depending on the luminance of the film viewer used and observation environments, but is basically the same. In contrast, an image displayed on a display fluctuates when displayed on a different type of display, or when the display undergoes a change in state or a change with time, or when there are other factors. The fluctuation could cause a false diagnosis. Therefore, in making a diagnosis from interpretation of an image displayed on a display, the display has to be kept in a given state to keep the displayed image in a given, proper state. However, even when the state of a displayed image is kept constant, the appearance of the displayed image is varied depending on observation environments.
For that reason, DICOM (Digital Imaging and Communication in Medicine, a standard for transmission of medical image data, waveform data, and the like) prescribes that a diagnostic image can be displayed on a display only after performing gradation correction using GSDF (Grayscale Standard Display Function). The standard also prescribes that gradation correction should be performed by taking into account not only the luminance of the display but also observation environments including the luminance of peripheral light. However, gradation correction in consideration of the luminance of peripheral light narrows the dynamic range of a medical image displayed when the contrast is lowered, and this works against the fact that a wide dynamic range of shades is required in a medical image.
On the other hand, when a display screen (polarization plate) of a display does not have the AG property (when the display screen is untreated), the display screen is large in specular reflectivity (about 4%) and therefore has less ability to avoid reflection of light of a viewer or an external object on the screen while an image obtained resembles a transmission image on a film which is vivid and sharp and which has no glare. In a medical monochrome LCD for interpreting subtle shades, to obtain an image resembling a transmission image on a film which is vivid and sharp and which has no glare takes precedence and therefore the display screen of the display is stripped of AG property. Reflection of light of a viewer or an external object on a display screen can be reduced by shutting out peripheral light, but this is not enough to completely eliminate reflection on the display screen because light of the display screen itself is reflected by the viewer of the display screen. In clinical environment, in particular, viewers usually wear lab coats and that much more likely to cause reflection on the screen.
This problem could be solved by giving a display screen of a display an anti-reflection (AR) property.
An anti-reflection (AR) film that has conventionally been used to impart the AR property is a multilayer film having layers of transparent thin films of metal oxides. Plural transparent thin films are used in order to prevent reflection of light in as wide a wavelength range as possible within the visible light range. Such transparent thin films of metal oxides are formed by evaporation, for example, chemical vapor deposition (CVD) and physical vapor deposition (PVD). Vacuum evaporation or sputtering which is one of physical vapor deposition methods is particularly employed to form those films. In the evaporation methods, an AR film obtained is low in specular reflectivity (0.5% or less) and therefore has an excellent ability to avoid reflection of light of a viewer or the surroundings on the display screen. However, the cost is high, and the original color of the displayed image is impaired by coloring due to the AR film, which is a serious problem. To elaborate, an AR film formed by one of those evaporation methods to have an average reflectivity of 0.5% or less, in particular, 0.4% or less, in a wavelength range of 450 nm to 650 nm is excellent in avoiding reflection on the display screen while the film has a poor reflection characteristic on the long wavelength side and the short wavelength side (especially the short wavelength side), resulting in poor color neutrality. Because of the poor color neutrality, light reflected by the AR film is heavily colored in reddish purple to bluish purple, and thus the. display quality is degraded. One more problem in employing evaporation or sputtering is that the manufacture method produces AR films in batches and therefore is low in productivity while being high in cost.
Application is another conventional way to obtain the AR property, and an AR film can be created by applying inorganic particles or polymers once or more to form a single layer or multiple layers. Application is a low-cost method because a large continuous area of a film can be formed by roll-to-roll. However, an AR film manufactured by application is about 1 to 2% in specular reflectivity and the average reflectivity exceeds 1%, which presents a serious problem of insufficient AR ability. Although some conventional application methods have been successful in obtaining an average reflectivity of 0.5% or less, light reflected by an AR film that is formed by application is heavily colored in reddish purple when the color of the reflected light is calculated from the reflection spectrum or when a real sample of the AR film is observed visually. Accordingly, the display quality is degraded as is the case for an AR film formed by evaporation. In addition, since the reflected light is colored heavily, a slight fluctuation in thickness of the anti-reflection layer leads to a color shift great enough to be recognized visually (see JP 11-6902 A).
For that reason, the AR property is not given to a display screen of a medical display for interpreting subtle shades, in particular, a medical monochrome display.